Return ErrSingular for singular matrix solves

[kortschak]

Fixes issue #30.

[coveralls]

Coverage decreased (-0.0%) when pulling 2409b42 on solve into 2a243e3 on master.

[btracey]

Why do LQ and QR panic if not full rank? I see that there is a possibility to check if not full rank, just wondering what the rule is.

[btracey]

Because of the special casing in solve, it would be nice to have the m > n and m < n cases as well.

[kortschak]

Not all matrices have an inverse; if a matrix does not have an inverse it is said to be singular. If a matrix is rank-deficient then it is singular.

[jonlawlor]

Does https://github.com/gonum/matrix/blob/master/mat64/lu.go#L224 need to be changed as well?

[coveralls]

Coverage increased (+0.48%) when pulling d002f59 on solve into 2a243e3 on master.

[kortschak]

That code is not called when the matrix is singular if using the Solve func. If the client is using the LU type directly, they can check IsSingular and deserve a panic if they don't.

[btracey]

LGTM